Recently, a content in which audio signals have been recorded in more than two channels has become more and more popular. For example, a movie content, in which audio signals have been recorded in six channels, is now available on a DVD.
Audio signals are usually supposed to be output through the same number of loudspeakers as their number of channels. For example, FIG. 1 illustrates a set of loudspeakers 11 through 16, which are arranged so as to surround a listener 17 and output six-channel audio signals. Specifically, a left channel (L) loudspeaker 11, a center channel (C) loudspeaker 12, a right channel (R) loudspeaker 13, a left surround channel (LS) loudspeaker 14, a right surround channel (RS) loudspeaker 15 and a low frequency effect (LFE) channel loudspeaker 16 are illustrated in FIG. 1.
The frequency range of the audio signal output through the LFE channel loudspeaker 16 is one tenth or less of those of the audio signals output through the other loudspeakers. And the LFE audio signal is sometimes counted as a “0.1 channel” audio signal. That is why the loudspeaker system shown in FIG. 1 is often called a “5.1 channel surround loudspeaker system”. In this description, however, the LFE audio signal is also counted as a one channel audio signal and the term “5.1 channel” will not be used herein.
When a content with six channel audio signals is broadcast as a TV program, the broadcaster sometimes converts the six channel audio signals into two channel audio signals before transmitting the program. This is done because the broadcaster wants that program to be viewed and listened to on an analog TV set with only two loudspeakers. Such processing for decreasing the number of channels of the audio signals is called “down mixing”. A TV set with two loudspeakers can output audio through its two loudspeakers based on the two-channel audio signals received.
Meanwhile, there are audio devices with more than two loudspeakers. The larger the number of loudspeakers through which the audio can be output, the greater the degree of existence added to the video. That is why the audio is preferably output through as many loudspeakers as possible independently of each other. For that reason, it has become more and more commonplace that a device that has received two-channel audio signals performs pseudo surround processing for generating pseudo channel data in more than two channels according to its own output performance.
A normal down mixing method is represented by the following Equations (1) and (2):Ldm=KLL×Lm+KLC×Cm+KLR×Rm+KLLS×LSm+KLRS×RSm+KLLFE×LFEm  (1)Rdm=KRL×Lm×KRC×Cm+KRR×Rm+KRLS×LSm+KRRS×RSm+KRLFE×LFEm  (2)
In Equations (1) and (2), Ldm denotes a left output signal generated, Rdm denotes a right output signal generated, Cm, Lm and Rm denote the center, left and right signals of the original audio signals, LSm and RSm denote left surround and right surround signals of the original audio signals, and LFEm denotes the low frequency effect signal of the original audio signals. By these Equations (1) and (2), six-channel audio signals (i.e., M=6) are down-mixed into two-channel audio signals (i.e., N=2). On receiving the left and right output signals Ldm and Rdm, a TV set with two loudspeakers outputs these audio signals through the respective loudspeakers.
The coefficients by which Cm, Lm, Rm, LSm, RSm and LFEm are multiplied in Equations (1) and (2) are as follows. The coefficients (A1) are called “left mixing coefficients” and the coefficients (A2) are called “right mixing coefficients”.
(A1): KLL=1.0, KLC=0.707, KLR=0.0, KLLS=−0.707, KLRS=−0.707, and KLLFE=0.0,                (A2): KRL=0.0, KRC=0.707, KRR=1.0, KRLS=0.707, KRRS=0.707, and KRLFE=0.0,        
The mixing coefficients are set to be these values in order to obtain a pseudo surround channel signal and a pseudo center channel signal as represented by the following Equations (3) and (4):Rdm−Ldm=−Lm+Rm+1.414×(LSm+RSm)  (3)Rdm+Ldm=Lm+1.414×Cm+Rm  (4)
According to Equation (3), the device that has received the left and right output signals Ldm and Rdm can obtain a pseudo boosted surround channel signal (LSm+RSm) by subtracting Ldm from Rdm. On the other hand, according to Equation (4), the device that has received the left and right output signals Ldm and Rdm can obtain a pseudo boosted center channel signal (Cm) by adding Ldm to Rdm. That is to say, by making the simple calculations by these Equations (3) and (4), the device can generate a pseudo center channel signal and a pseudo surround channel signal based on the two-channel output signals Ldm and Rdm and can eventually reproduce audio in four channels in total.
Patent Documents Nos. 1 to 3 disclose a technique for changing the settings of coefficients (or parameters) to be used for a down-mixing audio mixing device to down-mix six-channel audio signals into two-channel audio signals.
On the other hand, Patent Document No. 4 discloses an audio mixing device that maintains a predetermined multi-channel mixing direction and signal energy. According to this document, multi-channel input signals are down-mixed into output signals in response to left and right channel mixing coefficients ml and mr generated so that the signal energy and predetermined direction of the input signal are substantially maintained in the output signal.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 6-165079    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-241853    Patent Document No. 3: PCT International Application Japanese National Phase Publication No. 2001-518267    Patent Document No. 4: PCT International Application Japanese National Phase Publication No. 2005-523672